Operation of natural gas (NG) pipelines at high pressures in the dense-phase region is becoming attractive due to findings of NG fields in harsh environments and in remote areas, such as in Alaska and in the Northern Territories of Canada, where the transportation costs of a conventional pipeline may be prohibitive. The dense-phase is defined as the region where the gas pressure is higher than the cricondenbar. The cricondenbar is the maximum pressure at which two phases such as liquid and gas can coexist. Operation in the dense-phase region offers a means to reduce overall transmission cost and hence encourage the building of new systems in remote and harsh areas. Operating pipelines in the dense-phase permits transportation of raw gases, which typically contain varying amounts of natural gas liquids (NGL) from heavier gas components.
A challenge of dense-phase transportation of gas is related to gas quality monitoring at remote locations. It has been suggested that one of the main specifications of the gas mixture quality in the dense-phase is its cricondentherm. The cricondentherm is defined as the warmest temperature at which a liquid may be formed in a gas mixture. In a general sense, warmer cricondentherms may indicate the presence of heavier gas components which may be of concern from a liquid dropout perspective. Liquid dropout along a pipeline results in not only a loss of a gas component, but may pose operational problems along the gas delivery system. Challenges of developing such a gas mixture quality monitoring program include: 1. challenges related to the appropriate kind of detection equipment and system required, 2. appropriate sampling system techniques, and 3. characterization of C6+ fractions if a gas chromatograph (GC) based system is used.
Current techniques for gas quality management for gas mixtures in the dense-phase include: a) Near-Infrared (NIR) methods, b) condensate collection methods, and c) methods based on analysis of gas composition and use of an appropriate equation of state to determine the cricondentherm.
NIR spectroscopy has been used in remote sensing and in hostile environments. The concept is based on the Lambert-Beer law for absorption of NIR radiation. The absorbance is linearly proportional to the path length of a specific component in the gas mixture and its respective concentration in the mixture. However, typical NIR methods suffer from the drawback of not being able to distinguish between C3-C5 components. Typical NIR methods also cannot determine nitrogen concentration in the gas mixture, as nitrogen does not have absorption bands. NIR systems are also by and large expensive and typically require an extensive calibration program. As well, this method typically cannot be used for on-line measurements.
The condensate collection method is based on the determination of the quality of condensate formed at a certain pre-selected pressure and temperature, which may be agreed upon between the supplier and the buyer. The basis of this method is that the gas mixture is allowed to cool through an isobaric, adiabatic or isothermic process, and the condensates formed are collected and measured by weight against the sample flow. This method can be used on-line. However, careful weighing of the collected condensate is key in achieving good results, and the setup is typically elaborate and expensive.
Methods based on compete analysis of gas composition, for example using a GC, requires an appropriate equation of state to determine the cricondentherm. Unfortunately, equations of state often have inherent uncertainties in calculating the dewpoint, and results may vary depending on the equation of state or equation parameters used. Such methods also face the problem of proper sampling as NGL (i.e., the heavier components) will most likely be dropped out in the sample stream to the gas analyzer. In fact, most industrial GCs only analyze to C6, the heavier components being typically assumed. The drawback is that the dewpoint is heavily influenced by small (e.g., ppm) levels of the heavier components which may not be analyzed by the GC. Typically, the real cricondentherm is at a warmer temperature than that calculated via composition from a GC. The result is that the gas may be richer than thought and as a consequence there may be unwanted liquids formed. Typically, the accuracy required for the GC-EOS method to be accurate is to determine each of the C6+ components to better than 10 ppm. Current GC technologies under field conditions require significant capital and maintenance in a C9+ analyzer and sampling system to achieve this.
It would be desirable to provide a method and system for monitoring gas mixtures that can be used for gas mixtures in the dense-phase region that addresses some of these challenges.